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Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Range Of Performance Cycle From 2014-2016 |  |  |  |  |
Accrued Expense |  |  |  |  |
Percentage of target payout - minimum actual |  |  |  |  | 34.00%
Percentage of target payout - maximum actual |  |  |  |  | 76.00%
Performance Cycle 2015 to 2017 |  |  |  |  |
Accrued Expense |  |  |  |  |
Percentage of target payout - minimum actual |  |  | 145.00% |  |
Percentage of target payout - maximum actual |  |  | 147.00% |  |
White Mountains |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |
Vesting Period |  |  | 3 years |  |
Target Performance Share Activity |  |  |  |  |
Beginning of period (in shares) | 40,616 | 50,575 | 50,515 | 80,353 | 80,353
Shares paid or expired (in shares) | 0 | 0 | (23,186) | (30,838) |
New grants (in shares) | 0 | 0 | 14,105 | 17,510 |
Assumed forfeitures and cancellations | 0 | (256) | (818) | (16,706) |
Ending of period (in shares) | 40,616 | 50,319 | 40,616 | 50,319 | 50,515
Accrued Expense |  |  |  |  |
Accrued expense at the beginning of period | $ 27.2 | $ 31.2 | $ 45.8 | $ 42.4 | $ 42.4
Payments and deferrals | 0.0 | 0.0 | (28.4) | (21.6) |
Assumed forfeitures and cancellations | (0.1) | (0.4) | 0.1 | (9.4) |
Expense recognized | 7.8 | 7.0 | 17.4 | 26.4 |
Accrued expense at the end of the period | 34.9 | $ 37.8 | 34.9 | $ 37.8 | $ 45.8
Additional compensation cost that would be recognized if all outstanding performance shares vested | $ 20.4 |  | $ 20.4 |  |
Sirius Group |  |  |  |  |
Target Performance Share Activity |  |  |  |  |
Ending of period (in shares) |  | 2,195 |  | 2,195 |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The amount of performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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- Definition The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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